Travel Reference
In-Depth Information
core—the innermost of the three largest shells of earth structure—has never
been observed and never can be, Jules Verne fantasies aside. Yet we know a
great deal about it, and much of this information comes from the way and the
rate at which earthquake waves more through the earth. The core, unlike the
overlying mantle, is liquid, but liquid in a hellish way. It is under such high
pressure that although we technically classify it as a liquid, it is a type of liq-
uid that cannot exist on the surface. The composition of the core is metallic
(it is composed of iron and nickel), and its contact with overlying mantle
(which is "solid") must be one of the more interesting places in the solar sys-
tem. Complex interactions that occur at this core-mantle interface have
enormous ramifications for the rest of the planet. The discovery that the core
is made of hot liquid metal showed that it is the source of the magnetic field,
and the perturbations, eddies, convection currents, or other types of move-
ment within the spinning core may account for the magnetic reversals. Per-
haps irregularities or gigantic phase interactions between this liquid core and
the overlying, solid mantle region somehow trigger the phase changes. Be-
cause these regions are located thousands of miles beneath our feet, the evi-
dence on how and why a polarity reversal takes place is never direct.
Scientists have long known about reversals. It was not until the 1960s,
however, that the implications of this discovery for calibrating geological
time
became apparent. It was then that geologists began sampling thick piles
of lava flows on the edges of volcanoes. Because each individual lava flow
could be accurately dated using potassium/argon techniques, scientists were
able to record a relatively precise series of ages for the flows. Each dated flow
was then sampled for its
paleomagnetic
direction. To the surprise of the inves-
tigators, not only could individual normal and individual reversed directions
be detected, but
many
of these reversals in magnetic field were observable.
Geologists soon realized that the present, "normal" magnetic field direction
has existed only for about the last half-million years. Prior to that, the field
was "reversed" relative to its present pole directions. As ever-older piles of
lava were sampled, it became clear that the interval between reversal
episodes was irregular but generally quite long—on the order of hundreds of
thousands to millions of years.
